In positioning systems based on satellite positioning, a positioning receiver attempts to receive signals of at least four satellites in order to detect the position of the positioning receiver and the time data. An example of such a satellite positioning system is the GPS system (Global Positioning System) comprising a plurality of satellites orbiting the globe according to predefined orbits. These satellites transmit positioning data, on based of which the position of a satellite can be defined at each moment of time, in case the exact time data used in the satellite positioning system is known in the positioning receiver. In the GPS system, the satellites transmit a spread spectrum signal which is modulated with a code that is individual for each satellite. Thus, the positioning receiver can distinguish signals transmitted by different satellites from each other by using a reference code corresponding to the satellite code generated locally in the positioning receiver.
A drawback with such positioning systems based on satellite positioning is often the fact that a signal transmitted by a satellite is strongly attenuated when arriving to the positioning receiver, wherein it is very difficult to distinguish the signal from the background noise. The signal can be attenuated inter alia due to climatic conditions and obstacles, such as buildings and surrounding grounds in the routing of the signal. Also, the signal can wander to the positioning receiver through a plurality of different routes which causes so-called multipath propagation and aggravates the synchronizing of the positioning receiver to a wished signal because the transmitted signal arrives to the receiver through different routings, for example straight from the satellite (line-of-sight) and, in addition to this; reflected. Due to this multipath propagation the same signal is received as a plurality of signals with different phases. It is particularly difficult to perform positioning inside a building, because a building itself strongly attenuates the signal transmitted by satellites and, on the other hand, multipath propagation can be even stronger since possibly reflected signals coming for example through a window are not necessarily as attenuated as signals coming straight through the roof. In this case, the receiver can make erroneous interpretations about the time of flight and the positioning of the satellite during the moment of transmission, inter alia due to said increase in the signal time-of-flight caused by multipath propagation
In the mobile communication network MN (FIG. 1) every cell has an individual identifier CGI (Cell Global Identity). For example in the GSM system the cell global identity CGI usually comprises the following four parts:                mobile country code MCC,        mobile network code MNC,        location area code LAC, and        cell identity CI.        
When the base station BS, BS′, BS″ has an omnidirectional antenna the coverage area of the base station constitutes one cell. However, base stations BS, BS′, BS″ in which directional antennas are used each sector of the directional antenna can define one cell. Thus, the base station BS, BS′, BS″ may in fact constitute more than one cell and each cell can be identified by the individual identifier CGI of the cell.
Thus, every cell can be identified on the basis of the cell global identity CGI of this cell. The serving cell, that is the cell of the base station BS, BS′, BS″ through which the wireless communication device MS communicates with the mobile communication network MN at a time, transmits to the mobile communication device the cell global identity CGI of the serving cell. The cell global identity can be transmitted, for example, via a control channel of the mobile communication network MN. Thus, inter alia in connection with a cell handover the wireless communication device MN can detect the cell change on the basis of the change in the cell global identity CGI if the cell identity CI is available.
In FIG. 1 the cells are depicted as circles CE but it is obvious that in practice the shapes of the cells are not exactly circles. The base station of the cell can be assumed to be located at the center of the cell when the base station constitutes one cell BS, BS′, BS″. The wireless communication devices which have positioning means such as a GPS receiver attached can send information related to the cell global identity of the serving cell to the base station of the serving cell. For example, the positioning receiver informs the current position to the wireless communication device at intervals, e.g once in every second. The wireless communication device sends the position information and the cell global identity through the base station of the serving cell to a server in which the data base DB is formed. The server can then calculate the radius and the center point of the cell in question by using the positioning information related to the cell and/or to the base station of the cell. The accuracy of the calculation depends inter alia on the number of data pairs i.e. position data and cell global identity and how close or how far each position data are to/from each other. On the other hand, when there are many data pairs for a particular cell a new data pair does not have a great effect to the radius and the center point of the cell. Therefore, if the data base contains lots of data pairs in which the position data does not differ a lot, the calculation result may be significantly distorted. The operator of the server may also require a fee for using the information of the data base.
There does not need to be a separate server for the data base DB but each wireless communication device can form and update a data base of its own. In this alternative the wireless communication devices can use the positioning data from the positioning receiver of the wireless communication device. It is also possible that wireless communication devices exchange positioning data and cell global identity with each other. When the data base is located in the wireless communication device the user does not need to pay any fees for using the data base.
It is also possible that the data base is located in the wireless communication network.
The cell global identity CGI can be utilized to assist the positioning e.g. in the following manner. In the wireless communication device MS the information transmitted by the base station BS, BS′, BS″ is received, from which information inter alia on the cell global identity CGI or parts of it is found out. According to this identity it is detected whether any information related to the position of this particular cell and/or the base station BS, BS′, BS″ is stored in the memory of the wireless communication device. If no positioning data in accordance with the cell global identity is found in the memory, the necessary positioning data is searched from the database DB. The database may be located in the wireless communication device, in a network such as a mobile communication network, etc. If the positioning data is stored in the mobile communication network MN, for example in each base station BS, BS′, BS″, in the mobile switching center MSC, or in the GPRS packet network, the wireless communication device MS transmits to the mobile communication network MN a request to transmit the positioning data of the base station and/or the cell in question to the wireless communication device MS. As a response, the base station transmits the positioning data of the base station and/or the cell and, if necessary, other auxiliary data in accordance with the satellite positioning system as well, such as the orbit parameters and the almanac data of the satellites. The transmitted information is received in the wireless communication device MS, wherein at least the positioning data of the base station and/or the cell is stored in the memory of the wireless communication device MS. In addition, the received parts of the cell global identity CGI of the cell are stored, which can then be used as an index to the positioning data stored into the memory.
The radius R and the center point of the cell can be calculated on the basis of the positioning data related to the cell in question.
After the position of the serving cell and/or base station is known in the wireless communication device MS, it is possible to utilize this positioning data of the cell and/or the base station in the positioning by setting it as the default position of the positioning receiver. According to this positioning data the wireless communication device is informed of the approximate position of the wireless communication device on the globe. On the basis of the time data transmitted by the base station it is possible to estimate in the wireless communication device which satellites are above the horizon, that is, visible as seen from the wireless communication device. Next, the wireless communication device can attempt to search for the signals of these visible satellites. Positioning can thus be performed in a manner known as such by first using this default position and by precisioning the positioning on the basis of the signals received from the satellites SV1 to SV4.
The positioning receiver receives information transmitted by satellites and performs positioning on the basis of the received information. For the positioning, the receiver must receive a signal transmitted by at least four different satellites to find out the x, y, z coordinates and the time data. The received navigation information is stored in a memory, wherein this stored information can be used to find out e.g. the positioning data of satellites.
FIG. 1 shows, in a principle diagram, positioning, by means of a signal transmitted from four satellites SV1, SV2, SV3, SV4 in a wireless communication device MS comprising a positioning receiver. In the GPS system, the satellites transmit positioning data as well as time data, on the basis of which the positioning receiver can perform calculations to determine the current position of the satellite. These positioning data and time data are transmitted in frames which are further divided into subframes (not shown). In the GPS system, each frame comprises 1500 bits, which are divided into five subframes of 300 bits each. Since the transmission of one bit takes 20 ms, the transmission of each subframe thus takes 6 s, and the whole frame is transmitted in 30 seconds. The subframes are numbered from 1 to 5. In each subframe 1, e.g. time data is transmitted, indicating the moment of transmission of the subframe as well as information about the deviation of the satellite clock with respect to the time in the GPS system.
The subframes 2 and 3 are used for the transmission of positioning data. The subframe 4 contains other system information, such as universal time, coordinated (UTC). The subframe 5 is intended for the transmission of almanac data of all the satellites. The entity of these subframes and frames is called a GPS navigation message which comprises 25 frames, i.e. 125 subframes. The length of the navigation message is thus 12 min 30 s.
One drawback in prior art systems is that the data of a cell in the data base may be saturated due to numerous amounts of positioning data. This has the effect that new positioning data does not change the calculated parameters (radius, center point) of the cell. For example, if the data base is updated quite often by a wireless communication device using positioning data from a small area, it may have more effect to the calculation result compared with positioning data updated less often and from a larger area by another wireless communication device. For example, if the user of the wireless communication device is walking, the distance between successive position fixes does not change a lot compared with a situation in which the user e.g. drives a car i.e. the traveling speed of the wireless communication device affects to the positioning data of the data base.